Method for preventing the bulking of activated sludge

ABSTRACT

A method for preventing the bulking of activated sludge, which comprises adding to the activated sludge a water-soluble cationic polymer obtained by a reaction of a dialkylamine, ammonia and epichlorohydrin, in an amount of from 0.05 to 25 parts by weight relative to 100 parts by weight of the dried solid content of the activated sludge.

FIELD OF THE INVENTION

The present invention relates to a method for preventing the bulking ofactivated sludge. More particularly, an abnormal growth of activatedsludge whereby the sludge swells due to excessive proliferation ofSphaerotilus or molds in the activated sludge or for some other reason,is called "bulking", and the present invention relates to a method forpreventing the occurrence of such bulking due to an abnormal phenomenonof microorganisms.

Sewerage, night soil and various organic industrial waste waters areusually treated by an activated sludge method. It is known that duringthe treatment by this method, the microorganism phase in the activatedsludge undergoes a change in response to the change in the flow rate ofthe inflow waste water or the change of the organic substances in theinflow waste water. If such a change of the microorganism phase isabrupt, a so-called bulking phenomenon occurs, where the sludge swells,the sedimentation tends to be poor, the sludge rises towards the watersurface, the overflow of the sludge increases, the treating capacitydecreases and, in an extreme case, the treatment becomes impossible.When this phenomenon takes place, the sludge tends to be whitish, verylight and hardly sedimentable, and its BOD reducing efficiency tends tobe extremely low.

SVI (Sludge Volume Index) is used as an index to determine whetheractivated sludge is properly functioning. This SVI is expressed by thevolume occupied by the sedimented sludge when 1 g of activated sludgewas subjected to sedimentation for 30 minutes. The SVI of a properlyfunctioning activated sludge is from 50 to 150, particularly not higherthan 100. Whereas, the SVI of the activated sludge which has undergonebulking is likely to be as high as from 300 to 400. Once such bulkinghas taken place, it is not easy for the activated sludge to recover itsproper function, and in the worst case, it will be necessary to replacethe sludge. In the case of such replacement in a plant, it will benecessary to cease the operation until the fresh sludge is cultured andactivated to a proper level, and such replacement seriously affects theproduction plan of the plant.

DESCRIPTION OF THE RELATED ART

Methods for preventing the bulking phenomenon of activated sludge havebeen proposed, for instance, in Japanese Examined Patent PublicationsNo. 14274/1983 and No. 14275/1983.

Japanese Examined Patent Publication No. 14274/1983 discloses a methodwherein at least one dithiocarbamate having a specific structure isdissolved in water and added to activated sludge. However, this methodis intended solely for a case where the bulking has been caused by anabnormal proliferation of filamentous fungi, and this method is noteffective when the bulking is caused by any other factors. Besides,according to this method, it takes at least four days until the effectsbecome distinctive even when the damage to the activated sludge isrelatively slight, and in some cases, it takes ten days or more for theeffects to be distinctive. Further, there is an additional drawback thata large amount of the agent must be added everyday continuously. Themethod disclosed in Japanese Examined Patent Publication No. 14275/1983,comprises adding to waste water a composition containing one or moreamino acids such as valine, leucine, isoleucine, glutamic acid,phenylalanine and tyrosine, as active ingredients, so that the activeingredients are eventually incorporated to the activated sludge.According to this method, the composition containing these activeingredients is required to be added continuously for at least 3 hours ata rate of from 0.5 to 3 kg/m³ to the inflow waste water, and there is adrawback that it takes at least 24 hours until the effects will beobtained.

The bulking phenomenon of the activated sludge is caused not only by theabnormal proliferation of pilamentous fungi such as Sphaerotilus,Thiothrix, Asperqillus and Penicillium, but also by the formation ofhighly viscous jelly-like substance in the activated sludge particularlyduring the winter time, whereby the sludge tends to be hardlysedimentable and likely to exhibit a bulking phenomenon. In the lattercase, it is said to be effective to add ZnCl₂ or a cationicpolyacrylamide polymer flocculating agent to the waste water toflocculate the activated sludge. However, according to the experimentsconducted by the present inventors, it has been found that according tothis method, the activated sludge is apt to contain air bubbles in anaeration tank, and the activated sludge containing such air bubbles hasvery poor capability for the treatment of the waste water and does notshow distinctive effects for the prevention of the bulking phenomenon.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method forpreventing the bulking phenomenon of activated sludge, particularly atrouble attributable to an abnormal phenomenon of microorganisms such asthe bulking due to the abnormal proliferation of filamentous fungi andthe bulking due to the formation of highly viscous substance in thewinter time, whereby the treating operation will be easy and the SVIreducing effect can be regained in a short period of the time.

The present invention provides a method for preventing the bulking ofactivated sludge, which comprises adding to the activated sludge awater-soluble cationic polymer obtained by a reaction of a dialkylamine,ammonia and epichlorohydrin, in an amount of from 0.05 to 25 parts byweight relative to 100 parts by weight of the dried solid content of theactivated sludge.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described in detail with reference tothe preferred embodiments.

In the method of the present invention, a water-soluble cationic polymerobtained by a reaction of a dialkylamine, ammonia and epichlorohydrin isadded to activated sludge. As the dialkylamine, dimethylamine,diethylamine, diprophylamine, methylethylamine, methylpropylamine andethylpropylamine may be mentioned.

Ammonia may be added in a very small amount relative to thedialkylamine. The amount may be selected within a range ofammonia/dialkylamine=0.0001 to 0.6 by molar ratio, preferably 0.001 to0.1 by molar ratio.

For the reaction of the dialkylamine, ammonia and epichlorohydrin, theepichlorohydrin is used in substantially the same molar amount as thetotal molar amount of the dialkylamine and ammonia, and the reaction isconducted by using a closed type reactor equipped with a stirrer underan inert atmosphere by maintaining the internal temperature of thereactor within a temperature range of from 30° to 100° C.

More specifically, an aqueous solution containing from 20 to 70% byweight of a dialkylamine and ammonia is charged in a closed-type reactorequipped with a stirrer, a reflux condenser, a thermometer, etc., theatmosphere in the reactor is replaced by nitrogen gas, and whilecontrolling the internal temperature of the reactor not to increase toomuch under stirring, epichlorohydrin is added continuously orintermittently. After the completion of the addition of epichlorohydrinand ammonia to the reactor, an oxide or hydroxide of an alkali metal oralkaline earth metal is added to the reactor in an amount of from0.00001 to 0.01 mol relative to 1 mol of epichlorohydrin, and thereaction system is maintained within a temperature range of from 30° to60° C. for a few hours, whereby the molecular weight of the resultingpolymer can be increased. Further, if this reaction product ismaintained in the form of an aqueous solution under a nitrogen gasatmosphere at a temperature of about 30° C., the reaction to increasethe molecular weight proceeds slowly for about 2 months. If air oroxygen is introduced during this period, the reaction to increase themolecular weight ceases to proceed.

The molecular weight of the polymer as the reaction product can bejudged by the intrinsic viscosity [η] obtained by calculation from themeasurement of its solution in a 2 M KBr aqueous solution. In the methodof the present invention, it is preferred to employ the polymer havingan intrinsic viscosity [η] of at least 0.05, more preferably at least0.1, most preferably at least 0.2.

If the amount of the water-soluble cationic polymer is too small, theobject of the invention can not be achieved. On the other hand, if theamount is excessive, the polymer will be discharged out of the systemtogether with the waste water. Therefore, the amount is selected withina range of from 0.05 to 25 parts by weight relative to 100 parts byweight of the dried solid content of the activated sludge. A range offrom 0.1 to 10 parts by weight is preferred.

For the addition of such a water-soluble cationic polymer to theactivated sludge, an aqueous solution of the water-soluble cationicpolymer is prepared, and any one of the following methods or anycombination thereof may be employed:

(1) A method wherein the aqueous solution is mixed into the inflow wastewater, and the mixture is sent to an aeration tank;

(2) A method wherein the aqueous solution is directly added to anaeration tank or a sedimentation tank where the bulking of activatedsludge is taking place; and

(3) A method wherein the aqueous solution is added to the sludge forrecycling.

When the water-soluble cationic polymer is added to the activated sludgewithin the above-mentioned range, no increase in SVI is usually observedfor at least 2 month, although such depends upon the removal rate of theexcess sludge. Whenever a sign for an increase in SVI is observed, thewater-soluble cationic polymer may be added at that time within theabove-mentioned range.

The method of the present invention is as described above, and exhibitsthe following remarkable effects. Thus, its value for industrialapplication is extremely high.

(1) According to the method of the present invention, an increase in SVIof activated sludge can be prevented within a short period of timesimply by adding the water-soluble cationic polymer to the activatedsludge treatment system.

(2) According to the method of the present invention, no swelling of theactivated sludge in an aeration tank takes place, and it is possible tominimize the sedimentation volume, to maintain the activated sludgeconcentration at a high level and to remarkably improve the effects forreducing BOD.

(3) According to the method of the present invention, the sedimentationand separation of the activated sludge are extremely simple also in asedimentation tank. Besides, the volume of the sediment (sedimentationvolume) can be substantially reduced, whereby excess activated sludge ishardly formed, and frequent removal of excess activated sludge andconsequential incineration treatment thereof will not be required.

Now, the present invention will be described in further detail withreference to Examples. However, it should be understood that the presentinvention is by no means restricted to such specific Examples.

EXAMPLE 1

Waste water having COD of 380 ppm was supplied to four aeration tankseach having a capacity of 370 m³ at a rate of 35 m³ /hr per tank, andthe concentration of activated sludge was adjusted to 3,500 ppm (as thedry solid content relative to the waste water). Further, the waste waterin the aeration tanks was adjusted so that the ratio of COD:nitrogenelements:phosphorous element was 100:5:1, and the pH was 7. Then, airwas blown into each aeration tank at a rate of 800 liters/hr.

The liquid treated as above, was sent to a sedimentation tank having acapacity of 500 m³, and the sludge was permitted to settle naturally.The supernatant liquid and the concentrated liquid of activated sludgewere separated, and the supernatant liquid was discharged to a river.The concentrated liquid of activated sludge was returned as recyclingsludge to the aeration tanks at a rate of 80 m³ /hr. Out of theconcentrated liquid of activated sludge, 1 m³ /hr was taken out asexcess activated sludge, to which a flocculating agent was added toflocculate the sludge, followed by centrifugal dehydration to obtain adehydrated cake having a water content of 90%. The above described wastewater treament was continued for a long period of time, whereupon SVI ofthe activated sludge in the aeration tanks reached 320.

A polymer which was a reaction product of dimethylamine, ammonia andepichlorohydrin (molar ratio ofdimethylamine/ammonia/epichlorohydrin=100/1/100) and which had anintrinsic viscosity of 0.26 as measured in a 2 M KBr aqueous solution,was dissolved in water to obtain an aqueous solution having a polymerconcentration of 30% by weight, as a treating solution. To the wastewater supplied to the aeration tanks at the above-mentioned rate, theabove treating solution was added in an amount of 300 kg as the dryweight of the polymer solid content over a period of 4 hours. After thecompletion of the addition of the treating solution, SVI of theactivated sludge in the aeration tanks was investigated and found tohave sharply dropped to 87. Further, the concentration of the activatedsludge in the sedimentation tank following the aeration tanks increasedsubstantially from 7,500 ppm prior to the addition of the treatingsolution to 12,100 ppm after the addition. Further, no change wasobserved in the change of the dissolved oxygen concentration and in thereduction of BOD due to the addition of the treating solution.

For one month after the addition of the above treating solutionaccording to the method of the present invention, no rising of thesedimentation surface level of activated sludge in the sedimentationtank was observed, and there was no necessity for the treatment ofexcess sludge. Accordingly, the amount of the dehydrated cake to beincinerated decreases.

COMPARATIVE EXAMPLE 1

From an aeration tank of Example 1, activated sludge having SVI of 320and a solid concentration after drying of 3,500 ppm, was sampled in abeaker.

A polymer which was a reaction product of dimethylamine, ammonia andepichlorohydrin (molar ratio of the three components=100/1/100) andwhich had an intrinsic viscosity of 0.03 as measured in a 2 M KBraqueous solution, was dissolved in water to obtain an aqueous solutionhaving a polymer concentration of 30% by weight, as a treating solution.

The treating solution was added to the above sludge in a various amountwithin a range of from 50 to 400 ppm, and the reduction of SVI of theactivated sludge was observed. Even when the above treating solution wasadded to the sampled activated sludge within the above range, thesedimentation speed of the sludge was slow, and SVI did not decreasebeyond 160.

COMPARATIVE EXAMPLE 2

In the same manner as in Comparative Example 1, the reduction of SVI ofthe activated sludge was observed by using as the treating solution,four types of aqueous solutions of commercially available cationicpolyacrylamides i.e. KP355 (manufactured by Dia Flock K.K.), Dia ClearMK5000 (trade name, manufatrued by Mitsubishi Chemical IndustriesLimited), Sun Flock C450 (trade name, manufactured by Sanyo Kasei K.K.)and Kuricrock (trade name, manufactured by Kurita Kogyo K.K.) and byvarying the respective amounts of addition.

Substantially the same behavior was observed in all of the above fourtypes of treating solutions. Namely, a satisfactory flock forsedimentation started to from when the dry weight of the solid contentin each treating solution was about 50 ppm relative to the activatedsludge, and no change was observed in the formation of the flock forsedimentation within a range of from 100 to 400 ppm. Further, in everycase, SVI of the activated sludge after the addition of the treatingsolution did not decrease beyond 120.

I claim:
 1. A method for preventing the bulking of activated sludge,which comprises adding to the activated sludge a water-soluble cationicpolymer having an intrinsic viscosity of at least 0.05 as measured in a2 M KBr aqueous solution obtained by the reaction of a dialkylamine,ammonia and epichlorohydrin, in an amount of from 0.05 to 25 parts byweight relative to 100 parts by weight of the dried solid content of theactivated sludge.
 2. The method for preventing the bulking of activatedsludge according to claim 1, wherein the water-soluble cationic polymeris selected from those having an intrinsic viscosity of at least 0.1 asmeasured in a 2 M KBr aqueous solution.
 3. The method for preventing thebulking of activated sludge according to claim 1, wherein thedialkylamine is dimethylamine, diethylamine, dipropylamine,methylethylamine, methylpropylamine or ethylpropylamine.
 4. The methodfor preventing the bulking of activated sludge according to claim 1,wherein the water-soluble cationic polymer is added in an amount of from0.1 to 10 parts by weight relative to 100 parts by weight of the driedsolid content of the activated sludge.